The use of integrated circuits (ICs) during the last few decades is nothing short of fantastic. These miniature ICs are now used in a multitude of devices ranging from computers to toasters. These devices are typically constructed of a silicon wafer with impurities that create transistors precisely placed on the wafer. An arrangement of metal and insulating layers overlies the wafer for electrically connecting the transistors to one another. The overlying layers are typically between 50 angstroms and a few microns thick. The thickness and uniformity of the layers determine the efficiency of the chip and the application to which it is applied.
The manufacture of chips, although requiring the utmost in cleanliness and precision, has been refined to produce millions of devices in a short period of time. However, a continuing concern is producing efficacious devices that operate properly. As stated, one indication of proper operation is the thickness of the various semiconductor layers of the device. However, providing accurate measurement of the thickness is extremely difficult.
One efficient method is destructive as a chip is cut and viewed from the cut side. It is common for a manufacturer to try to control layer thickness by closely controlling every element affecting manufacture of the chips, such as pressure, temperature and humidity, and by destroying a few chips to verify the thickness of their layers.
Another fairly recently developed technique for chip layer thickness determination involves the use of laser beams to first excite a sample chip with a first optical pulse and then monitor the subsequent relaxation process with a weaker second optical pulse. However, this process can acoustically examine only a single point at a time. Because of this, chip inspection can be a lengthy operation, often causing delays in the production line. A need exists for equipment that can more quickly and accurately determine chip layer thickness.
It is therefore an object of the present invention to provide method and apparatus for efficiently determining the thickness of thin layers of a material.
It is another object of the present invention to provide method and apparatus capable of determining thickness of a thin layer of material along a line.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, a line sensing device for laser acoustic inspection of thin film specimens comprises a laser emitting pulses of light with a first beamsplitter outputting a first portion of the pulses of light and a second portion of the pulses of light. A second beamsplitter receives the second portion of the pulses of light and outputs a third portion of the pulses of light and a fourth portion of the pulses of light. A first optical delay line receives the first portion of the pulses of light for introducing a predetermined time delay into the first portion of the pulses of light. A second cylindrical lens receives the delayed first portion of the pulses of light and directs the delayed first portion of the pulses of light onto a thin film specimen along a line. A first cylindrical lens receives the third portion of the pulses of light for directing the third portion of the pulses of light onto the thin film specimen so that the third portion of the pulses of light contact the thin film specimen along the line. A second optical delay line receives the fourth portion of the pulses of light for introducing a predetermined time delay into the fourth portion of the pulses of light and directs the delayed fourth portion of the pulses of light to a photorefractive crystal. A third cylindrical lens receives reflected pulses of light from the thin film specimen and directs the reflected pulses of light to the photorefractive crystal. A fourth cylindrical lens collects light from the photorefractive crystal and transmits the collected light from the photorefractive crystal to a linear photodiode array allowing inspection of the thin film specimen along a line.
In another aspect of the present invention, and in accordance with its principles and purposes, a method of sensing a line along a thin film specimen comprises the steps of emitting ultrafast laser pulses; splitting the ultrafast laser pulses into a first portion, a second portion, a third portion and a fourth portion of the ultrafast laser pulses; delaying the first portion of the ultrafast laser pulses by a first predetermined period of time; directing the delayed first portion of the ultrafast laser pulses at the thin film specimen along a line; delaying the fourth portion of the ultrafast laser pulses by a second predetermined period of time; directing said delayed fourth portion of said ultrafast laser pulses to a photorefractive crystal; collecting pulses of light reflected from the thin film specimen and directing the reflected pulses of light to the photorefractive crystal; and collecting light from the photorefractive crystal and transmitting the collected light to a linear photodiode array allowing inspection of the thin film specimen along a line.